The apparatus and method of this invention lie in the field of offshore operations of various kinds and are directed primarily to the drilling and maintenance of wells located beneath the ocean floor. They are more particularly directed to the provision of apparatus suitable for supporting drilling or servicing equipment used in connection with wells which are located at great water depths.
Since the inception of exploration for oil beneath the ocean floor a great variety of drilling rigs have been devised and built for use in varied situations. In the earlier part of the period drilling was usually confined to fairly shallow water and it was possible to sink shafts or pilings into the ocean bed, anchor them, and then mount a working structure which extended above the surface of the water. The working platform was located above the crests of the waves to minimize disturbance. Since the jackets (or towers) and platforms were very heavy the shafts or legs were in compression and had to be very large and strong to prevent column failure; thus, the structures were quite expensive. Moreover the shafts were continually subjected to wave action which caused considerable beam bending. The platforms were permanently attached and were subject to all of the storms which occurred, and added wind load to wave load applied to the shafts. As drilling moved to deeper water, the legs or shafts became longer and both compressive and bending forces became more servere. The use of this system in depths beyond a few hundred feet becomes extremely difficult and costly.
Submersible type rigs have also been used for many years. In this type, a complete structure is built up initially, including a hollow base section which may be filled with air so that the rig can be floated to the site of operations where the base is ballasted with sea water and sinks to the ocean floor. Once it is placed in position it is virtually the same as the built-up structure previously mentioned and is subject to all of the same problems.
Another type which has been used extensively is the floating rig. This includes ships or barges which are located over the drill site and which require massive mooring systems to combat the forces of winds and waves which continually tend to move them out of position. They may use mooring cables or propellers operating in different directions, or both. Since the rigs are practically never stationary they require very complicated and expensive drilling apparatus. Another type of floating rig employs very large floats or pontoons which remain beneath the surface and structural members rising above the surface to support working platforms. They are thus somewhat less responsive to wind and wave forces. While the floating rigs can obviously be used in deep water, far beyond the range of the other types, they still suffer the disadvantage of being constantly in motion and requiring expensive drilling apparati.